1. Field of the Invention
The present invention is generally related to the manufacture of composite structures. In particular, the present invention is related to computer-controlled fiber placement apparatus used to apply fiber tow to a laydown surface, usually about a rotating mandrel, to form a workpiece. More particularly, the present invention is especially suitable for, but not limited to, providing optimum fiber tow tension, controlling the velocity of the fiber tow, and precisely monitoring fiber travel as the fiber tow is despooled and ultimately applied to a laydown surface.
2. Background of Related Art
Fiber tow placement/winding machines typically place, or wind, a plurality of fibers, fiber tows, or filaments that have been impregnated with a thermosetting resin in a preselected pattern or weave, which is positioned upon a laydown surface mounted on a rotating mandrel. The laydown surface or workpiece may be cylindrical or, more likely than not, of an irregular shape. Regardless of the geometry the final workpiece is to have, it is generally imperative that the dimensions, contours, and overall structural quality of the workpiece meet or exceed very strict manufacturing and integrity requirements.
Most fiber tow placement/winding machines have a plurality of spools containing fibers or tows in which the fiber is de-spooled in order for it to be precisely positioned about the workpiece by a computer-controlled head located on a robotic arm. It is important that each fiber, of which there can be 32 or more individual spools of fiber, as it is being de-spooled and positioned simultaneously by the head, be wound about the workpiece at a preselected tension which is optimum for the particular fibers being used and the geometry of the particular laydown surface or workpiece being constructed. Such tensions can typically range from 0.25 pounds of force to approximately 1.0 pounds of force. Therefore, there is a long-standing need in the composite manufacturing industry to precisely control fiber tension as much as possible, or at least maintain the tension of the fiber within an acceptable range, as the fiber is being de-spooled from its individual spool and subsequently positioned about the workpiece.
Additionally, the tensioning system may be called upon to hold the fiber in a stand- by or stationary mode should that particular fiber not be needed on a given portion of the workpiece. Fiber tensioning systems are also called upon to take up any slack in the fiber slowly when tension is first applied to the fiber by the tensioning system so as not to unduly stress or break the fiber. Furthermore, fiber tensioning systems ideally detect malfunctions in the delivery of fiber, report them to a master computer or controller, and, if deemed appropriate, release the tension on the fiber. Preferably, fiber tensioning systems should be able to monitor fiber usage and detect a nearly empty spool condition and report such information to a master controller whereupon the operator can replace the nearly empty spool with a minimum of disruption. Therefore, it can be appreciated that there is a need within the industry for a fiber tensioning system that not only properly and precisely maintains the tension of a multitude of fibers within a narrow range of force in an economical manner but also reliably performs a variety of monitoring, reporting, and fault-detection tasks as well.
Fiber tensioning systems, including those used in computerized fiber placement/winding machines, often use a plurality of pulleys, or redirect rollers, mounted on a support structure which guides and supports the fiber as it is being de-spooled, or paid out, as it travels toward the computer-controlled head along a preselected path. Typically, the spool of fiber is mounted on a spool holder which is driven by a processor- controlled electric motor.
A fiber tension system incorporated within a fiber placement machine is disclosed within U.S. Pat. No. 5,223,072xe2x80x94Brockman et al., which is incorporated herein. The Brockman et al. ""072 patent depicts a load cell being mounted to a redirect roller to sense the tension of the fiber, which in connection with a CNC controller system and a local microprocessor then controls the torque being imparted upon the spool by a drive motor. Furthermore, the CNC controller system and local processor uses position and velocity feedback information from the drive motor and incorporates a velocity loop as well as a force loop.
With respect to using pulleys, or redirect rollers, to manipulate fiber path in order to maintain a preselected tension, an article published in the August 1997 issue of PCIM magazine beginning on page 90, which is incorporated herein, describes a tension control system used in the manufacturing of nano-technology fiber sensors for the biotechnology industry. The described system includes the use of a dancer arm having a redirect roller at a free end of the arm and a low friction air cylinder attached at a midpoint along the arm. The computer-controlled, closed-loop feedback system allows fiber tension to be maintained by making automatic adjustments to an electric pressure regulator to increase or decrease the pressure being supplied to the cylinder in order to reposition the arm, thereby changing fiber tension during the manufacturing process thereof.
An objective of the present invention includes providing an accurate, reliable, and low-cost fiber tow tensioning system. Another objective is to reduce if not eliminate time-consuming calibration requirements and to provide a tensioning system suitable for use not only in any fiber placement machine but for use in any machine or process where tension and control of a fibrous element is required. It is a yet further objective to provide a fiber tension system that neither measures force, including electrically controlled force, nor requires analog sensors and associated circuitry which are subject to drift. Another objective of the present invention is to provide a fiber tension system which eliminates the need to control fiber spool holder torque, thereby allowing the use of inexpensive stepper motors to drive the fiber spool holder. Another objective is to provide a fiber tensioning system which detects a broken tow or other malfunction and reports such to a main controller and, if deemed appropriate, ceases tensioning. An additional objective is to provide a fiber tensioning system that takes up fiber slack slowly when tension is first applied, detects a nearly empty spool, determines fiber usage, as well as monitors fiber speed. These and other objectives and benefits of the present invention will become apparent upon reviewing the present specification in its entirety.
The present invention provides an apparatus and method for controlling tension of one or more fiber tows and preferably comprises a support frame having a spool holder, a motor for driving the spool holder, a dancer arm having a free end with a redirect roller mounted thereon and a fixed end pivotally mounted to coact with an angular-position sensor, a low-friction pneumatic cylinder mounted to the dancer arm and the support frame, a second redirect roller mounted to coact with a second angular-position sensor, a controller and associated circuitry to control the position-controlled tensioner system.
Preferably, a microcontroller mounted on a single printed circuit board (PCB) having signal buffering circuitry to control four individual tensioners is provided. The PCB preferably plugs into a STD-style backplane which allows communication between several microcontrollers, thereby allowing the control of at least 32 individual tensioners. Communication between a fiber placement machine main, or master, controller and each tensioner controller is also provided. Preferably, the motors driving the spool holders are stepper motors driven by respective stepper drivers. The microcontroller controls the stepper motor so as to feed or not feed fiber in order to maintain the dancer arm in a preselected position, usually its center position. The air cylinder mounted to each dancer arm is connected to a common pneumatic pressure regulator so that all tensioners in the same orientation receive the same pneumatic pressure. The tension on each fiber is thus controlled by monitoring its respective dancer arm""s position, as being sensed by the angular-position sensor co-located with the dancer arm at its pivot point and accordingly activating the stepper motor to cause the fiber to allow the dancer arm to return to its neutral position. Additionally, forces due to gravity and the pneumatic cylinder acting upon the dancer arm and hence the fiber tow contribute to maintaining fiber tension as well. The second angular-position sensor coacting with its respective redirect roller provides fiber speed information, thereby allowing better control of the dancer arm position, and estimation of current spool diameter to better track fiber usage and to detect any irregularities or malfunctions of fiber travel.
Because each microcontroller preferably controls four tensioners, each micro controller is directly aware of both adjacent neighboring tensioners with respect to the other pair of tensioners within its direct control. Furthermore, the PCB""s and the backplane are designed to allow the fiber speeds of the other tensioner""s neighbors to be readily available to the main fiber placement machine controller should the detection of a broken tow fiber occur in any one of the tensioners. Additionally, the control logic detects if a fiber fails to de-spool and will automatically deactivate the respective stepper motor and monitor fiber travel thereafter. Thereafter, the control logic will then cause the stepper motor to slowly reinitiate tensioning if the detected problem appears to have cleared itself. An optional clutch, which is automatically engaged upon the stepper motor being activated and which automatically disengages when the stepper motor is deactivated, may be provided. Thus, the optional clutch provides a mechanism in which to selectively couple and decouple the spool holder from the stepper motor drive train and assists in further reducing fiber tension upon the controller sensing a de-spooling problem.